Modularly constructed electrical devices can be adapted to specific tasks by inserting different modules into them. Thus for example a telephone switching system can be adapted to a specified number of extensions by inserting a sufficient number of modules. Typically the modules communicate via a rear panel with one another, as is shown schematically in FIG. 1. On the rear panel a distinction is made between bidirectional lines 1 and unidirectional lines 2a and 2b. On each side both a transmitter 4a and 5a and also a receiver 4b and 5b respectively are connected to a bidirectional line. Unidirectional lines are driven on one side by a transmitter 6a and 7a and on their other side are connected to a receiver 6b or 7b. 
If two modules with the same electrical interface communicate with one another it is necessary to avoid one line being simultaneously operated by two transmitters. A bidirectional line on the rear panel can connect two corresponding plug-in connections of the two modules with one another since by activating or deactivating the transmitter for this plug-in connection care is taken that a maximum of one transmitter is driving the line.
Unidirectional lines will typically be crossed on the rear panel so that, although the corresponding plug-in connections on the two modules could possibly be driven by two transmitters, they are however linked to two other plug-in connections of the other modules in each case, as is shown in FIG. 1 for lines 2a and 2b. Thus in FIG. 1 line 2a connects transmitter 6a with receiver 6b. Correspondingly line 2b connects transmitter 7a with receiver 7b. 
If a new generation of modules is now developed the number of bidirectional and unidirectional connections is fixed by the rear panel. When identical modules are used a bidirectional pair of lines cannot be used for two unidirectional signals since in this case two transmitters would drive the same line or one line would connect the inputs of two receivers with one another. Without changing the rear panel and in accordance with the conventional technology only one bidirectional signal can be sent via a pair of crossed wires on the rear panel since by crossing the lines the signal is not sent and received via the corresponding plug-in connectors.
If the new generation of modules is to be more powerful then as a rule a higher data rate is also to be transmitted via the rear panel. This could be done by providing a larger number of connections or by converting from unidirectional lines into bidirectional lines. Conventionally therefore a redesign of the rear panel has been necessary in which for example crossed lines have been replaced by uncrossed lines. However replacing the rear panel leads to the electrical device having to be temporally taken out of service and thus the replacement of the rear panel has resulted in long periods of interruptions in service. By contrast modules are designed so that they can be changed even when the device is in operation.